A nonsense mutation in the GPIIb heavy chain (Ser 870-->stop) impairs platelet GPIIb-IIIa expression

Expanding the Mutation Spectrum Affecting αIIbβ3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbβ3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbβ3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbβ3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and β3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Mutations in the β3 gene giving rise to type I Glanzmann thrombasthenia in two families in Portugal

Glazzmann thrombasthenia is an inherited bleeding syndrome in which an absence of platelet aggregation is associated with quantitative or qualitative deficiencies of the alphaIIbbeta3 integrin. We now describe biochemical and molecular studies on two Portuguese families where platelets lack both surface and intracellular pools of alphaIIbbeta3. DNA extraction was followed by PCR-SSCP analysis of all exons and intronic boundaries in the alphaIIb and beta3 genes. Migration abnormalities were found for PCR fragments encompassing exon 12 (family 1) and exon 10 (family 2). For patient 1, there was a homozygous G to T transition at position 1846 which resulted in a stop codon at codon 616 in the beta3 gene. For patient 2, direct sequencing revealed a homozygous 1347C insert which led to a stop codon at codon 444 in the beta3 gene. For both patients a single mutated allele was inherited from each parent. Evidence is accumulating that nonsense mutations leading to a truncated beta3 may be a frequent cause of type I Glanzmann thrombasthenia in the Iberian peninsula.

Glanzmann Thrombasthenia Associated with a 21-Amino Acid Deletion (Leu817-Gln837) in Glycoprotein IIb due to Abnormal Splicing in Exon 25

We report a novel genetic defect in a Japanese patient with type I Glanzmann thrombasthenia. The glycoprotein (GP) Ilb complementary DNA (cDNA) from platelet messenger RNA had a 63-base pair deletion in the 5' boundary of exon 25, resulting in an in-frame deletion of 21 amino acid residues (Leu817-Gln837) in the calf-2 domain. The deleted region was present in the genomic DNA, but the splice acceptor site (AG) of exon 25 was mutated to AC, leading to the use of an AG sequence in the middle of exon 25 as an abnormal cryptic splice acceptor site. The effect of this deletion on protein synthesis was further analyzed. Mutant GPIIb-IIIa complexes were not detected on the surfaces of cells cotransfected with cDNAs of mutant GPIIb and normal GPIIIa. Mutant pro-GPIIb was detected in cell lysates and was coimmunoprecipitated with an anti-GPIIb-IIIa complex antibody. Immunostaining demonstrated that the mutant pro-GPIIb colocalized with an endoplasmic reticulum protein, calnexin, within the cells. These results indicate that complex formation was not completely prevented and that impairment of the subsequent transport was the major reason for the defect in cell surface expression. The data suggest that the GPIIb calf-2 domain is important for intracellular transport of GPIIIb-IIIa complexes.

Major mutations in calf-1 and calf-2 domains of glycoprotein IIb in patients with Glanzmann thrombasthenia enable GPIIb/IIIa complex formation, but impair its transport from the endoplasmic reticulum to the Golgi apparatus

The crystal structure of integrin alphavbeta3 comprises 3 regions of contact between alphav and beta3. The main contact on alphav is located in the beta-propeller while calf-1 and calf-2 domains contribute minor interfaces. Whether or not contacts between calf-1 and calf-2 domains of glycoprotein (GP) IIb (alphaIIb) and GPIIIa (beta3) play a role in GPIIb/IIIa complex formation has not been established. In this study we analyzed the effects of 2 naturally occurring mutations in calf-1 and calf-2 domains on GPIIb/IIIa complex formation, its processing, and transport to the cell membrane. The mutations investigated were a deletion-insertion in exon 25 located in calf-2 and an in-frame skipping of exon 20 located in calf-1. Mutated GPIIb cDNAs were cotransfected in baby hamster kidney cells with normal GPIIIa (beta3) cDNA. Analysis by flow cytometry failed to demonstrate detectable amounts of GPIIb or GPIIb/IIIa complex on the surface of cells transfected with each mutation, but immunohistochemical staining revealed their intracellular presence. GPIIb was mainly demonstrable as pro-GPIIb by immunoprecipitation of cell lysates expressing each mutation. Differential immunofluorescence staining of GPIIb and cellular organelles suggested that most altered complexes were located in the endoplasmic reticulum. Homology modeling of normal GPIIb based on the alphavbeta3 crystal structure revealed similar contacts between alphav and beta3 and between alphaIIb and beta3. Introduction of the mutations into the model yielded partial disruption of the normal contacts in the corresponding domains. These data suggest that despite partial disruption of calf-1 or calf-2 domain, GPIIb/IIIa complex is formed but its transport from the endoplasmic reticulum is impaired.

A novel homozygous splice junction mutation in GPIIb associated with alternative splicing, nonsense-mediated decay of GPIIb-mRNA, and type II Glanzmann's thrombasthenia

This work reports the study of a patient suffering a bleeding disorder clinically diagnosed as Glanzmann's thrombasthenia (GT). Immunoblotting and flow cytometric analysis showed a low ( Collapse

Key Words Collapse

MESH Headings

• Alternative Splicing/genetics
• Codon, Nonsense
• Conserved Sequence/genetics
• Family Health
• Female
• Homozygote
• Humans
• Infant
• Mutation, Missense
• Platelet Glycoprotein GPIb-IX Complex/genetics
• Platelet Membrane Glycoproteins
• Point Mutation
• RNA Splice Sites/genetics
• RNA Stability/genetics
• RNA, Messenger/analysis
• Thrombasthenia/genetics

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Affiliation(s)

C González-Manchón Department of Pathophysiology and Human Molecular Genetics, Centro de Investigaciones Biológicas (CSIC), Madrid, Spain.
E G Arias-Salgado
N Butta
G Martín
R B Rodríguez
I Elalamy
R Parrilla
R Favier

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A novel (288delC) mutation in exon 2 of GPIIb associated with type I Glanzmann's thrombasthenia

This work reports the molecular genetic analysis of two patients who suffer mucocutaneous haemorrhages, prolonged bleeding time and failure of platelets to aggregate, either spontaneously or in response to agonists. The absence of platelet surface glycoprotein (GP)IIb-IIIa complexes confirmed the clinical diagnosis of Glanzmann's thrombasthenia (GT). Polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) analysis of exon 2 of GPIIb showed polymorphic bands caused by the homozygous deletion of a cytosine at position 288 relative to the translation start site. causing a shifting of the reading frame and appearance of a premature termination codon. The heterozygous relatives showed a reduced platelet content of GPIIb-IIIa, and a correlation was found between the levels of GPIIb mRNA and surface expression of GPIIb-IIIa complexes. Unlike other mRNAs carrying a nonsense mutation, (288Cdel)GPIIb does not force alternative splicing of GPIIb mRNA. As expected, co-transfection of Chinese hamster ovary (CHO) cells with cDNAs encoding GPIIIa and (288delC)GPIIb failed to enhance the surface exposure of GPIIIa. It is concluded that the (288delC)GPIIb mutation is responsible for the thrombasthenic phenotype of the patients. In addition, it has also been determined that heterodimerization of GPIIb-IIIa requires the integrity of exons 2 and 3 of GPIIb.

A Leu262Pro mutation in the integrin β3 subunit results in an αIIb-β3 complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, IIbβ3. In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant IIbβ3 may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable IIbβ3, findings consistent with type II GT. Genotyping of LD revealed 2 novel β3 mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among β integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Proβ3 cotransfected with wild-type IIb into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Proβ3 formed a complex with endogenous av and retracted fibrin clots similarly to wild-type β3. The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for IIbβ3 to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding β3 Leu262 may maintain β3 in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.

A Leu262Pro mutation in the integrin β3 subunit results in an αIIb-β3 complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, IIbβ3. In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant IIbβ3 may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable IIbβ3, findings consistent with type II GT. Genotyping of LD revealed 2 novel β3 mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among β integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Proβ3 cotransfected with wild-type IIb into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Proβ3 formed a complex with endogenous av and retracted fibrin clots similarly to wild-type β3. The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for IIbβ3 to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding β3 Leu262 may maintain β3 in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.

Truncation of Glycoprotein (GP) IIIa (▵ 616-762) Prevents Complex Formation With GPIIb: Novel Mutation in Exon 11 of GPIIIa Associated With Thrombasthenia

This work reports the molecular genetic study of a patient who suffered from Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (GP) IIb and GPIIIa genes showed the presence of a homozygous G1846→T transversion in exon 11 of GPIIIa that changes Glu616→Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in the proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the thrombasthenic phenotype of the proband. (1) We failed to detect mutations other than [T1846]GPIIIa in the coding region of both GPIIb and GPIIIa genes. (2) The G1846→T mutation was observed in either parent and a brother of the proband, but none of 100 unrelated individuals carried this defect. (3) Pulse-chase and immunoprecipitation analysis of GPIIb-IIIa complexes in cells transiently cotransfected with cDNAs encoding normal GPIIb and [T1846]GPIIIa showed neither maturation of GPIIb nor complex formation and surface exposure of GPIIb-▵GPIIIa. These observations indicate that the sequence from Glu616 to Thr762 in GPIIIa is essential for heterodimerization with GPIIb. Polymerase chain reaction-based analysis demonstrated the presence of normal levels of full-length GPIIIa-mRNA in the proband and in heterozygous relatives. In addition, a shortened transcript, with a 324-nucleotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mutations, [T1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon.

Truncation of Glycoprotein (GP) IIIa (▵ 616-762) Prevents Complex Formation With GPIIb: Novel Mutation in Exon 11 of GPIIIa Associated With Thrombasthenia

This work reports the molecular genetic study of a patient who suffered from Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (GP) IIb and GPIIIa genes showed the presence of a homozygous G1846→T transversion in exon 11 of GPIIIa that changes Glu616→Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in the proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the thrombasthenic phenotype of the proband. (1) We failed to detect mutations other than [T1846]GPIIIa in the coding region of both GPIIb and GPIIIa genes. (2) The G1846→T mutation was observed in either parent and a brother of the proband, but none of 100 unrelated individuals carried this defect. (3) Pulse-chase and immunoprecipitation analysis of GPIIb-IIIa complexes in cells transiently cotransfected with cDNAs encoding normal GPIIb and [T1846]GPIIIa showed neither maturation of GPIIb nor complex formation and surface exposure of GPIIb-▵GPIIIa. These observations indicate that the sequence from Glu616 to Thr762 in GPIIIa is essential for heterodimerization with GPIIb. Polymerase chain reaction-based analysis demonstrated the presence of normal levels of full-length GPIIIa-mRNA in the proband and in heterozygous relatives. In addition, a shortened transcript, with a 324-nucleotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mutations, [T1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon.

Prenatal diagnosis of Glanzmann thrombasthenia using the polymorphic markers BRCA1 and THRA1 on chromosome 17

Glanzmann thrombasthenia is an autosomal recessive bleeding disorder caused by mutations in the genes encoding platelet GPIIb or GPIIIa. Both genes map to chromosome 17q21 and polymorphisms within this chromosomal region have been identified. In the current study, prenatal diagnosis was performed for a family that already had one affected child, patient 1, who had a compound heterozygous mutation in GPIIb. At the time of prenatal diagnosis, the maternal GPIIb mutation had been identified but the paternal GPIIb mutation was unknown. By sequence analysis, the fetus was identified as a carrier of the mother's mutation. To determine the probability of the fetus inheriting the father's mutation, haplotype analysis of DNA samples from the fetus, mother, father and affected child were performed using polymorphic markers on chromosome 17q12-q21. These markers included polymorphisms within the thyroid hormone receptor alpha1 gene (THRA1), the breast cancer gene (BRCA1), GPIIb, GPIIIa, and an anonymous marker D17S579. Heterozygosity within the THRA1, BRCA1 and GPIIIa polymorphic markers predicted that the fetus carried the father's normal allele. Based on genetic linkage studies, no recombination was identified with any of the informative markers, and from the map distance between GPIIb and BRCA1 the accuracy of diagnosis was predicted to be >98%. The father's mutation was subsequently identified and direct sequence analysis of fetal DNA confirmed that the fetus did not inherit the fathers' mutant allele.

Glycoprotein IIb Leu214Pro Mutation Produces Glanzmann Thrombasthenia With Both Quantitative and Qualitative Abnormalities in GPIIb/IIIa

Glanzmann thrombasthenia is an inherited bleeding disorder due to a functional reduction or absence of platelet GPIIb/IIIa (αIIbβ3) integrin receptors. Based on a prolonged bleeding time and absence of platelet aggregation in response to physiologic agonists, a 55-year-old white man was diagnosed as having Glanzmann thrombasthenia. The patient's platelet fibrinogen level was ≈5% of normal. As judged by complex-dependent monoclonal antibody (MoAb) binding, surface expression of platelet GPIIb/IIIa receptors was less than 5.5% of normal, whereas the binding of an anti-GPIIIa specific MoAb (7H2) was ≈12% of normal. Immunoblot analysis of the patient's platelet lysates showed ≈35% of normal levels of GPIIIa, ≈30% of normal levels of GPIIb, and an abnormally migrating fragment of GPIIb. Biotinylation of the surface proteins on the patient's platelets followed by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed only GPIIb and GPIIIa subunits of normal size. Surface expression of platelet αvβ3 receptors was 192% of normal, suggesting that the patient's' defect was in GPIIb. Sequence analysis of the patient's GPIIb cDNA identified a T to C transition at nucleotide 643, predicting a Leu214Pro substitution. Direct sequencing of GPIIb exon 6 indicated that the patient is homozygous for the mutation. The nature of the Leu214Pro mutation was analyzed by expression in Chinese hamster ovary (CHO) cells. As judged by subunit-specific MoAb binding, surface expression of mutant receptors was ≈60% of normal, but these receptors were not recognized by the complex-dependent monoclonal antibodies, 10E5 and 7E3. In addition, mutant receptors pretreated with the ligand-induced binding site MoAb AP5 were not recognized by the activation-dependent MoAb PAC-1 and mutant expressing CHO cells did not adhere to immobilized fibrinogen. These data suggest that the Leu214Pro mutation in GPIIb disrupts the structural conformation, and either directly or indirectly, the ligand binding properties of the heterodimeric complex. This is in accord with studies from other integrins that have implicated a β-turn in a homologous region as important in ligand binding. Thus, the Leu214Pro mutation appears to produce the Glanzmann thrombasthenia phenotype by both qualitative and quantitative abnormalities. In addition, the mutation appears to confer susceptibility of the GPIIb subunit to proteolysis.

Glycoprotein IIb Leu214Pro Mutation Produces Glanzmann Thrombasthenia With Both Quantitative and Qualitative Abnormalities in GPIIb/IIIa

Glanzmann thrombasthenia is an inherited bleeding disorder due to a functional reduction or absence of platelet GPIIb/IIIa (αIIbβ3) integrin receptors. Based on a prolonged bleeding time and absence of platelet aggregation in response to physiologic agonists, a 55-year-old white man was diagnosed as having Glanzmann thrombasthenia. The patient's platelet fibrinogen level was ≈5% of normal. As judged by complex-dependent monoclonal antibody (MoAb) binding, surface expression of platelet GPIIb/IIIa receptors was less than 5.5% of normal, whereas the binding of an anti-GPIIIa specific MoAb (7H2) was ≈12% of normal. Immunoblot analysis of the patient's platelet lysates showed ≈35% of normal levels of GPIIIa, ≈30% of normal levels of GPIIb, and an abnormally migrating fragment of GPIIb. Biotinylation of the surface proteins on the patient's platelets followed by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed only GPIIb and GPIIIa subunits of normal size. Surface expression of platelet αvβ3 receptors was 192% of normal, suggesting that the patient's' defect was in GPIIb. Sequence analysis of the patient's GPIIb cDNA identified a T to C transition at nucleotide 643, predicting a Leu214Pro substitution. Direct sequencing of GPIIb exon 6 indicated that the patient is homozygous for the mutation. The nature of the Leu214Pro mutation was analyzed by expression in Chinese hamster ovary (CHO) cells. As judged by subunit-specific MoAb binding, surface expression of mutant receptors was ≈60% of normal, but these receptors were not recognized by the complex-dependent monoclonal antibodies, 10E5 and 7E3. In addition, mutant receptors pretreated with the ligand-induced binding site MoAb AP5 were not recognized by the activation-dependent MoAb PAC-1 and mutant expressing CHO cells did not adhere to immobilized fibrinogen. These data suggest that the Leu214Pro mutation in GPIIb disrupts the structural conformation, and either directly or indirectly, the ligand binding properties of the heterodimeric complex. This is in accord with studies from other integrins that have implicated a β-turn in a homologous region as important in ligand binding. Thus, the Leu214Pro mutation appears to produce the Glanzmann thrombasthenia phenotype by both qualitative and quantitative abnormalities. In addition, the mutation appears to confer susceptibility of the GPIIb subunit to proteolysis.

Illegitimate transcription: its use for studying genetic abnormalities in lymphoblastoid cells from patients with Glanzmann thrombasthenia

Glanzmann thrombasthenia is the most common inherited disorder of platelets that may induce severe bleeding complications. Molecular biology techniques have offered the possibility to assess the basis of this chronic haemorrhagic disease at the molecular level. However, the accessibility of mRNA in platelets is limited by the availability of the patient's blood samples and the relatively weak amount of this material in these cells. Taking advantage of the genetic phenomenon of illegitimate transcription, we have demonstrated that glycoprotein IIb and glycoprotein IIIa mRNA could be detected in lymphoblastoid cell lines issued from normal EBV-transformed lymphoblasts. We further analysed the sequences of the two glycoprotein transcripts in lymphoblastoid cell lines from two previously characterized patients presenting with Glanzmann thrombasthenia. The results showed that illegitimate transcripts presented similar molecular abnormalities to those found in platelets. These data demonstrated that the nucleotide sequences of illegitimate transcripts were identical to tissue-specific mRNA found in platelets. We applied this methodology to screen for the genetic defect in a new thrombasthenic patient, and found a homozygous nonsense mutation GCA-->TGA converting Arg8 to stop in the glycoprotein IIIa gene. This immortalized source of genetic material is therefore particularly useful for molecular genetic studies in inherited platelet disorders, avoiding repetitive and large blood samplings in frequently anaemic patients.

A Three Amino Acid Deletion in Glycoprotein IIIa Is Responsible for Type I Glanzmann's Thrombasthenia: Importance of Residues Ile325Pro326Gly327 for β3 Integrin Subunit Association

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by abnormal platelet aggregation due to a qualitative or quantitative defect of the glycoprotein (GP) IIb-IIIa complex (integrin αIIbβ3). We describe a new mutation in the GPIIIa gene responsible for type I GT in a consanguineous Algerian family. A discordance between phenotyping and genotyping of the GPIIIa-related HPA-1 platelet alloantigen system in three family members heterozygous for the disease suggested a genetic defect in the GPIIIa gene and a normal GPIIb gene. Sequence analysis of amplified genomic DNA fragments showed a 6-bp deletion in exon 7 of the GPIIIa gene resulting in the amino acid deletion/substitution (Ile325Pro326Gly327 → Met) and creating a new BspHI restriction site. Expression of the mutated integrin β3 subunit cDNA in Chinese hamster ovary cells showed that the cDNA gene was transcribed into a full-length β3 protein with an apparent molecular weight identical to wild-type β3 and accumulated as a single-chain molecule in the cell cytoplasm. The absence of heterodimeric complex formation of the mutant β3 protein with endogeneous αv was shown by immunoprecipitation experiments, intracellular immunofluorescent labeling, and a semiquantitative enzyme-linked immunosorbent assay using the αvβ3 complex-specific monoclonal antibodies LM609 and 23C6. Substitution of the methionine residue by a proline, present at position 326 of wild-type β3, did not restore the ability of the recombinant mutant β3 protein to associate with αv, suggesting that the Ile-Pro-Gly motif is located in a β3 domain important for integrin subunit interaction. The association of a BspHI restriction site with this newly identified mutation has allowed allele-specific restriction analysis of Algerian GT individuals and the identification of two new unrelated type I patients exhibiting the same mutation, suggesting that the described mutation might be significant in this population and that BspHI restriction analysis will provide a useful screening assay for antenatal diagnosis and genetic counselling.

A Three Amino Acid Deletion in Glycoprotein IIIa Is Responsible for Type I Glanzmann's Thrombasthenia: Importance of Residues Ile325Pro326Gly327 for β3 Integrin Subunit Association

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by abnormal platelet aggregation due to a qualitative or quantitative defect of the glycoprotein (GP) IIb-IIIa complex (integrin αIIbβ3). We describe a new mutation in the GPIIIa gene responsible for type I GT in a consanguineous Algerian family. A discordance between phenotyping and genotyping of the GPIIIa-related HPA-1 platelet alloantigen system in three family members heterozygous for the disease suggested a genetic defect in the GPIIIa gene and a normal GPIIb gene. Sequence analysis of amplified genomic DNA fragments showed a 6-bp deletion in exon 7 of the GPIIIa gene resulting in the amino acid deletion/substitution (Ile325Pro326Gly327 → Met) and creating a new BspHI restriction site. Expression of the mutated integrin β3 subunit cDNA in Chinese hamster ovary cells showed that the cDNA gene was transcribed into a full-length β3 protein with an apparent molecular weight identical to wild-type β3 and accumulated as a single-chain molecule in the cell cytoplasm. The absence of heterodimeric complex formation of the mutant β3 protein with endogeneous αv was shown by immunoprecipitation experiments, intracellular immunofluorescent labeling, and a semiquantitative enzyme-linked immunosorbent assay using the αvβ3 complex-specific monoclonal antibodies LM609 and 23C6. Substitution of the methionine residue by a proline, present at position 326 of wild-type β3, did not restore the ability of the recombinant mutant β3 protein to associate with αv, suggesting that the Ile-Pro-Gly motif is located in a β3 domain important for integrin subunit interaction. The association of a BspHI restriction site with this newly identified mutation has allowed allele-specific restriction analysis of Algerian GT individuals and the identification of two new unrelated type I patients exhibiting the same mutation, suggesting that the described mutation might be significant in this population and that BspHI restriction analysis will provide a useful screening assay for antenatal diagnosis and genetic counselling.